First and foremost, HEPA & ULPA Filters are different in definition and efficiency. HEPA Box are capable of removing 99.97% of contaminant particles 0.3 μm in diameter. Most standards also specify that HEPA filters must feature minimal pressure drop and maximum airflow when in operation. ULPA filters are closely related to HEPA filters but are even more efficient. ULPA filters are specified to remove 99.999% of contaminants 0.12 μm or larger in diameter.